1. Field
Apparatus and methods described in this document relate to frequency generators and methods for frequency generation. More specifically, the apparatus and methods relate to frequency generation using multiple oscillators.
2. Background
Tunable frequency generators are used in many different electronic devices. Wireless communication devices, for example, use frequency generators for upconversion of transmitted signals to intermediate and RF frequencies, and for downconversion of received signals to intermediate and baseband frequencies. Because operating frequencies vary, the generators' frequencies need to be tunable.
Frequency coverage required for multiple communication standards and multiple bands typically necessitates wide tuning range oscillators, such as voltage controlled oscillators (VCOs) and digitally controlled oscillators (DCOs). The extent of an oscillator's tuning range is one important performance parameter. It is often desirable to increase the tuning range, for example, in order to cover multiple bands.
Other performance criteria of tunable oscillators include phase noise performance, power consumption, and size. The different performance criteria are sometimes competing.
Conventional tunable oscillators may be tuned by applying a varying biasing voltage to a variable capacitor (varactor or varicap), and by switching capacitors in the oscillator inductance-capacitance (LC) tank. For a variety of reasons, the frequency range of a single oscillator obtained through these capacitance-varying techniques is limited. For this reason, multiple tunable oscillators may need to be used within the same device. Especially in the case of portable devices, such as cellular handset and other handheld communication devices, it is often desirable to implement an oscillator on the same integrated circuit (IC or chip).
Inductors (the “L” in the “LC”) occupy substantial area of a small IC. It is, of course, desirable to reduce the physical size of ICs. Locating two or more LC oscillators on the same IC therefore presents certain design difficulties. Thus, it is desirable to reduce the IC area occupied by the inductors of the multiple oscillators. Furthermore, it may be desirable to reduce the coupling between or among the inductors of the different oscillators built on the same IC.
Given the physical proximity of the inductors located on the same IC, however, can make substantial inductor-to-inductor coupling difficult to avoid. Such coupling may result in unwanted oscillation modes of a particular oscillator, in addition to the desired oscillation mode resulting from the resonance of the LC tank of the oscillator. It may be desirable to suppress such additional oscillation modes, so that the particular oscillator will generate frequencies based on its own LC tank.
Therefore, there is a need in the art for tunable oscillators with extended frequency range. There is also a need in the art for reducing the size of the IC packages containing multiple tunable oscillators. There is an additional need in the art for suppressing undesirable modes of oscillation in oscillators with non-trivial coupling between their inductors.